HYDROLOGIC RESPONSE OF A STEEP, UNCHANNELED VALLEY TO NATURAL AND APPLIED RAINFALL

Observations from natural rain storms and sprinkling experiments at a steep zero-order catchment in the Oregon Coast Range demonstrate the i mportance of flow through near-surface bedrock on runoff generation an d pore pressure development in shallow colluvial soils. Sprinkling exp eriments, involving irrigation of the entire 860 m(2) catchment at ave rage intensities of 1.5 and 3.0 mm/h, permitted detailed observation o f runoff and the development of subsurface saturation under controlled conditions. A weir installed to collect flow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi-steady irrigation. Three k ey observations demonstrate that a significant proportion of storm run off flows through near-surface bedrock and illustrate the importance o f shallow bedrock flow in pore pressure development in the overlying c olluvial soil: (1) greater discharge recovery during both the experime nts and natural rainfall at a weir installed approximately 15 m downsl ope of the weir at the base of the catchment, (2) spatially discontinu ous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associa ted with flow from weathered rock into the overlying colluvium during high-intensity rainfall. Data from natural storms also show that small er storms produce no significant runoff or piezometric response and po int to a critical intensity-duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction between flow in colluvium and nearsurface bedrock in governing patter ns of soil saturation, runoff production, and positive pore pressures.